The present invention relates to a fabrication process that produces dense carbonized carbon-carbon composites.
Recent advances in the field of aerospace technology have created a need for high strength, temperature resistant materials that possess the necessary properties needed to protect rocket combustion chambers and re-entry vehicles from the severe temperatures encountered within their environment. Re-entry vehicles are especially vulnerable to the stress and strain induced by their re-entry environment and require shapestable external surfaces capable of surviving those stresses. Existing carbon-carbon materials have proven somewhat effective as materials for application to re-entry vehicles and show adequate thermal stress performance. Carbon-carbon composites are being used for all three rocket nozzle stages of the MX missile. Carbon-carbon composites are being considered by the United States Air Force for use as manrated engine or cruise missile engine structures. Their ability to withstand 3500.degree. F. offers the promise and potential of extending range by 75% by the same amount of fuel, but limited to temperatures of less than 2500.degree. F. Finally, carbon-carbon composites are being considered for use as primary structures for large space stations and satellites.
The task of fabricating carbon-carbon composites is alleviated, to some degree, by the prior art techniques contained in the following U.S. patents:
U.S. Pat. No. 4,032,607 issued to Schultz on 28 June 1977;
U.S. Pat. No. 4,201,611 issued to Stover on 6 May 1980; and
U.S. Pat. No. 4,234,650 issued to Schieber on 18 November 1980.
The Schultz and Stover patents, incorporated herein by reference, are typical of the prior art which discloses impregnating a carbon fiber web with pitch by attempting to drive fluid pitch into the interstices of the web by high pressure, e.g. 15,000 psi, heating the impregnated web to harden the pitch and thereafter carbonizing the web at a temperature above 2000.degree. C.
The prior art methods of producing carbon-carbon composites involves impregnation of a reinforcing fiber array with pitch under pressure and then heating combination from outside. Although an applied pressure shifts pyrolysis equilibria so that a higher carbon yield is obtained, enough mass is still lost so that several impregnation and densification cycles are needed to produce material of suitably low porosity, and high density. While pressures of about 1000 psi result in reasonably open porosity, the higher pressures used in most prior art methods (e.g., 15,000 psi) produce a composite in which the pores tend to be closed, thus unfillable.
The Schieber patent, incorporated herein by reference, discloses a process of impregnating a carbon fiber felt at amospheric pressure with a fluid, carbonizable resin, i.e. phenol-formaldahyde resin, and then hardening the resin. Schieber then repeats this procedure on the same felt so that the density of the article increases.
Most of the prior art techniques follow the examples of Schultz and Stover. They provide a processing method involving several cycles of impregnation and carbonization densification under high autoclave pressure. This is very expensive and involves low yield and acute quality control problems. As mentioned above, the high-pressure prior art techniques tend to produce a composite in which the pores are closed. These techniques are then repeated in a series of cycles to increase the density of the final article.
The technique of Schieber is a more delicate approach, involving impregnating carbon fiber felt with resin at atmosphereic pressure and then hardening the resin. However, Schieber's process also involves a series of cycles to increase the density of the articles.
From the foregoing discussion, it is apparent that there currently exists the need for a new process that produces a suitably dense minimally porous carbonized carbon-carbon composite in only one or two cycles from petroleum pitch, coal-tar pitch, fractions obtained from either pitch, or resin impregnants. The present invention is directed towards satisfying that need.